This study will examine the postnatal changes in structure and function of mammalian cochlear neurons which underlie the maturation of hearing. The development of cochlear innervation patterns will be charted in an age- graded series of neonatal gerbils using standard in vivo and newly devised in vitro Horseradish peroxidase tract-tracing methods applied to intact and acutely-excised cochleae. Parallel in vitro studies of the development of cochlear neuron arbors in long-term organotypic cultures of the gerbil organ of Corti will examine the potential role of neuronal competition between developing cochlear afferents and efferents in sculpting a mature pattern of cochlear hair cell innervation. The ultrastructural events which guide the formation and pruning of cochlear neuron arbors with their target hair cells will be studied in both intact and cultured cochleae by serial reconstruction of selected HRP- labeled afferent terminals using high voltage electron microscopy (HVEM). Postnatal changes in the response properties of cochlear neurons to sound will be explored at specific sites within the cochlear spiral using a new spiral ganglion cell recording approach in neonatal gerbils. Over a quarter of a million children in this country are hearing impaired due to some form of congenital sensorineural hearing loss. The proposed studies should provide crucial normative data which are required for understanding the mechanisms by which disease alters the development of structure and function in the auditory periphery.